1. Field of the Invention
The present invention relates to a non-aqueous dispersion of hydrophilic particles that contain stabilized ascorbic acid, which is useful in the cosmetic, dermatological and/or veterinary fields, and methods for using these dispersion in the cosmetic treatment of the skin as well as for the preparation of a cream or ointment intended for the dermatological treatment of the skin and/or for veterinary treatment.
2. Background of the Art
Workers have long sought to stabilize ascorbic acid, or vitamin C, in suitable pharmaceutical forms to enable wider use of its beneficial properties.
Ascorbic acid has many known biological functions, such as the stimulation of collagen synthesis, the strengthening of skin tissues against external attack (UV radiation, pollution), reduction in loss of pigmentation, activity against free radicals and compensation for vitamin B deficiency. Some of these beneficial properties have been reported in particular by England and Seifter in the article "The bio-chemical functions of ascorbic acid" published in Ann. Rev. Nutri., 1986; 6, pp. 365-406.
However, the chemical structure (alpha-keto lactone) of ascorbic acid is very sensitive to the influence of environmental parameters such as light, oxygen and water (due to ascorbic acid's pH sensitivity and due to the presence of trace metals in water which may form chelates with the ascorbic acid). A heretofore unavoidable degradation of ascorbic acid in aqueous solution therefor occurs over time.
This problem has been addressed in a variety of ways in the art. For example, to reduce or delay the degradation of ascorbic acid in solution, U.S. Pat. No. 5,140,043 recommends stabilization by introducing ascorbic acid into aqueous-alcoholic solutions, formed of at least 80% water and having a pH below 3.5.
The high acidity of these solutions reduces or negates their utility in the cosmetic and/or pharmaceutical field. Repeated applications of such solutions could disrupt the equilibrium of the skin and might irritate, or even bum, the skin.
B. R. Hajratwala, in "Stability of Ascorbic Acid", published in the Revue Sciences Pharmacentiques on Mar., 15, 1905, teaches that ascorbic acid may be stabilized as an acidic aqueous solution by adding a surface-active agent which is an oxyethylenated sorbitan ester. In particular, Hajratwala states that a pH=3.4 and 25.degree. C., the addition of this agent reduced the rate of oxidation, and thus the rate of degradation, of ascorbic acid in solution. Hajratwala also teaches the use of a chelating agent (e.g., for monovalent cations) such as ethylenediaminetetraacetic acid (EDTA) and packaging under nitrogen, in the absence of light, to enhance the stability of an aqueous ascorbic acid solution.
Again, however, such an acidic aqueous solution if applied to the skin, would have the same drawbacks as those described above for acidic aqueous-alcoholic solutions. Furthermore, the stabilization provided would be insufficient.
Other ways of stabilizing ascorbic acid have been proposed, in particular by a coating technique (FR-A-1,600,826) or by granulation of ascorbic acid (JP-A-53-127,819) for the agriculture-foods industry.
However, these techniques tend to be expensive and may damage the ascorbic acid, for example during heating suggested for some of the processing, and/or may lead to compositions of poor cosmetic acceptability, as in the case of granules.
FR-A-1,489,249 discloses the use of metal salts of phosphorylated ascorbic acid, in particular magnesium ascorbylphosphate, in cosmetic compositions. The latter compound has an activity close to that of ascorbic acid, from which it is derived, but it has certain drawbacks which render its use on the skin less desirable. In particular, since magnesium ascorbylphosphate is only stable at basic pH (pH 8 to pH 9), it must be incorporated into a basic composition which may be an irritant to the skin (the pH of which is about 5.5).
U.S. Pat. No. 5,308,621 describes a composition for use in the transdermal administration of ascorbic acid (vitamin C) comprising a pharmaceutically acceptable carrier having 1 to 60% by weight of ascorbic acid in suspension within the carrier, the suspension of ascorbic acid comprising fine particles of ascorbic acid sized below 20 microns, preferably between 2 and 10 microns. The composition is formed by mixing the ascorbic acid into the carrier, heating up the mixture to dissolve the vitamin C, then cooling the solution to precipitate the vitamin C as small crystals. Typical carriers include polyhydric alcohols, alcohols, polyalkylene glycols, ointment bases such as petroleum jelly and lanolin, and the like. The preferred compositions are essentially water free, with less than about 0.5% by weight water.
U.S. Pat. No. 5,409,693 describes the use of ascorbic acid in the form of a fat-soluble fatty acid ester to treat sunburn and prevent sunburn damage to skin. The ascorbic acid derivative may be dispersed or dissolved in a pharmaceutically acceptable carrier.
U.S. Pat. No. 5,552,446 describes the use of a water in oil emulsion containing stabilized ascorbic acid. The emulsion contains an aqueous phase with a pH which is at most 3.5 (an acidic phase) in combination with particular emulsifying agents selected from dimethiconecopolyol or alkyldimethiconecopolyol. The stabilized ascorbic acid in the emulsion is suggested for use in the cosmetic, dermatological and/or veterinarian fields. The emulsion is taught to contain at least 60% by weight of water to prevent release of oil from the emulsion which would destabilize the emulsion. The ascorbic acid content is 0.5 to 5% by weight. With a maximum of 5% ascorbic acid it is limited because higher concentrations are desirable (see U.S. Pat. No. 5,140,043; column 3; line 22).
Consequently, none of the previous proposals have made it possible to overcome the technical problem associated with the instability of ascorbic acid in solution, in a pharmaceutical form which is suitable for the cosmetic and/or dermatological fields and at a cost which is compatible with industrial requirements.